We have measured the oxygen isotope effect on the electrical properties of
La0.65Ca0.35MnO3 under hydrostatic pressure up to 1.9 GPa. Special attentio
n is paid to the conductivity activation energy, E-a, above the insulator-m
etal transition temperature, T-c. E-a is a universal linear function of T-c
. The change of E-a by pressure and isotopic mass can be scaled to the chan
ge of T-c. These results can be qualitatively understood in terms of the do
uble exchange model and the adiabatic hopping of Jahn Teller polarons.